The "easy to prosecute for constructive possession" never comes to pass. It takes someone getting into their spotlight for other reasons and then the parts are scooped up as evidence and the charge added to the dogpiling to ensure a conviction for something that can stick.
Nobody seems to really give a damn. Because, not a problem on the national scale.
Compare the number of parts kits swept up in raids to the number of guns the ATF let walk across the border and we will then have real facts to contrast in the way of enforcement.
These kits sell on the premise of "sketchy ownership" "I have full auto gun parts and you don't." Same as "I have a switchblade in NYC." Has it stopped them yet?
You get stopped for suspicion of some other crime and then the knife is found. They aren't lining up people on the sidewalks and searching them. No real enforcement, no big deal. Lockbacks being prosecuted for gravity knife convictions have even been stopped now.
Parts kits have been selling since I first saw them in Shotgun News in the '80s. That's over 40 years now, and I was a newb then. It had been going on before that. So were sales of flats, Sten gun kits, etc. Common as dirt. Outlawing them was done to make them more desireable, and add fuel to the fire that weapons needed to be completely banned. Long term goal, to disarm America.
The NFA was originally written to ban handguns to all Americans. They got MG's silencers and a few short barrels, and went from there. Then the AWB but we let it sunset. Actual Patriots woke up and we got that rescinded. Look at the AR market now.
That ploy backfired on them.
Buy all the kits you want, it's your right under the 2d Amendment. For some of us, meh. We shot them in the service and we sure aren't going to feed one now that we have to pay for the ammo. It's like adopting a Great Dane when you are on Social Security - you might need that can of dog food for supper.